The FMC (Flexible Magnetic Coupler) is a partial discharge sensor whose working principle is based on a direct magnetic coupling with the cable conductor and shield.
 
It picks up a magnetic signal given by PD activity. His compact and robust design makes the FMC the optimal sensor for direct installation on cables and accessories such as joints and termination.
This sensor has a sensitive side which has to face to the object under test, while the other side (shielded) rejects the noise from the environment.
Although it performs particularly well on medium voltage cables, it can be used as well on high voltage cables, terminations or joints.
When other kind of sensors lacks in easiness of use or whenever an outage cannot be met, this sensor is a good choice.
In many practical cases we don’t have ground cables where other type of sensors could fit (i.e. HF CTs) and FMCs can make the difference between the possibility to perform a PD measurement or not.